A large-capacity power conversion apparatus for converting an AC voltage supplied from an AC power supply into a DC voltage, further converting the DC voltage into a pulse-width-modulated (PWM) voltage, and supplying the PWM voltage to a load has conventionally been known.
For example, as a power conversion apparatus for 400 W to 5 kW, a passive filter power conversion apparatus is available in which a reactor is connected to an AC power supply line, and an AC voltage obtained via the reactor is rectified by a voltage doubling/rectifying circuit in order to increase the power factor and reduce the harmonic component of the power supply (distortion of the power supply waveform).
FIG. 17 is a circuit diagram showing the arrangement of an inverter apparatus used in an air conditioner using an AC power supply voltage in 100V class, i.e., a conventional passive filter power conversion apparatus for controlling the ability of a refrigeration cycle drive motor.
As shown in FIG. 17, one terminal of a reactor Lin is connected to one terminal of an AC power supply Vin.
The other terminal of the reactor Lin is connected to the connection point between series-connected diodes DH and DL.
The series circuit made up of the diodes DH and DL is parallel-connected to the series circuit made up of diodes D1 and D2 and the series circuit made up of capacitors CH and CL.
The other terminal of the AC power supply Vin is connected to the connection point of the diodes D1 and D2 and to the connection point of the capacitors CH and CL called voltage doubling capacitors.
A smoothing capacitor CD is connected between the two terminals of the series circuit made up of the capacitors CH and CL.
The voltage across the smoothing capacitor CD is supplied to an inverter 50.
When the inverter 50 is connected to a load of about 1.8 kW, a reactor Lin having an inductance of 6.2 mH, voltage doubling capacitors CL and CH each having a capacitance of 360 pF, and a smoothing capacitor CD having a capacitance of 1,600 .mu.F are employed.
In the positive half cycle of the AC power supply Vin, the capacitor CH is charged via the diode DH; in the negative half cycle, the capacitor CL is charged via the diode DL.
The sum of the capacitor CH charge voltage and the capacitor CL charge voltage is applied to the smoothing capacitor CD, and thus the voltage twice the AC power supply Vin is supplied to the inverter 50.
The diode D1 forms a discharge circuit so as not to reversely charge the capacitor CH at the start of charge.
The diode D2 forms a discharge circuit so as not to reversely charge the capacitor CL at the start of charge.
The diodes DH, DL, D1, and D2, the voltage doubling capacitors CH and CL, and the smoothing capacitor CD shown in FIG. 17 constitute a conversion section (to be described later) according to the present invention. The inverter 50 constitutes an inversion section (to be described later) according to the present invention.
That is, this conversion section includes a voltage doubling/rectifying circuit 45 and the voltage doubling capacitors CH and CL.
When a compressor drive motor (not shown) serving as a load is driven using the conventional power conversion apparatus, a harmonic component is generated in the power supply, as indicated by the current value in FIG. 18.
FIG. 18 shows a current I (Lin) together with a current I (IEC) in class E standard by IEC (International Electrotechnical Commission). Comparing the current I (Lin) with the current I (IEC), the third harmonic component of I (Lin) exceeds that of I (IEC).
The third harmonic component can be reduced using a reactor having a larger inductance. In this case, however, the apparatus becomes bulky.
In the power conversion apparatus shown in FIG. 17, the power factor of the power supply is as relatively low as about 93%. As the load increases, an AC input current may increase, and the current value may reach a predetermined limit value. For this reason, the rotational speed and the like of the compressor drive motor (not shown) serving as a load are often limited.
FIG. 19 is a circuit diagram showing the arrangement of an inverter apparatus used in an air conditioner using an AC power supply voltage in 200V class, i.e., a passive filter power conversion apparatus for controlling the ability of a refrigeration cycle drive motor.
As shown in FIG. 19, the series circuit made up of diodes D3 and D4 is parallel-connected between the two terminals of the series circuit made up of diodes D1 and D2 to form a known full-wave rectifying circuit 40.
One terminal of an AC power supply Vin is connected to the connection point between the diodes D1 and D2, whereas the other terminal is connected to the connection point between the diodes D3 and D4.
A power factor improvement capacitor CP is connected between the two terminals of the series-connected diode circuit, and a smoothing capacitor CD is also connected between them via a reactor Lin and a reverse-flow prevention diode DB.
The voltage across the smoothing capacitor CD is supplied to an inverter 50.
The full-wave rectifying circuit 40 by the diodes D1, D2, D3, and D4, and the smoothing capacitor CD shown in FIG. 19 constitute a conversion section (to be described later) according to the present invention. The inverter 50 constitutes an inversion section (to be described later) according to the present invention.
That is, this conversion section includes the full-wave rectifying circuit 40 and the smoothing capacitor CD.
FIG. 20 shows the voltage and current waveforms of one cycle in the power conversion apparatus shown in FIG. 19.
As shown in FIG. 20, even a conduction angle of 110.degree. leads to a power factor of only 90%. Compared to an AC power supply voltage in 100V class, an AC input current hardly reaches a limit value for the same input power.
However, the power factor is lower than in the power supply in 100V class. To increase the power factor, a reactor having a larger inductance must be used, resulting in a bulky apparatus.